1. Field of Invention
This invention relates to microwave active susceptors, and particularly such susceptors suitable for use in the packaging and preparation of microwave food products, where the susceptor structure includes a fluid absorbent structure, which absorbs such fluids as water, moisture, oil, fat, grease, and the like.
2. Description of Related Art
The use of microwave energy revolutionized food preparation and has now become common place, both in businesses and in the home. However, the advantages associated with microwave usage are tempered with compromises to food appearance, texture, flavor, and sometimes ease of preparation. In particular, although microwave energy can be used to cook foods, it lacks the ability to provide a crisping effect to the bakery or breaded/fried foods. Accordingly, a majority of uses of the microwave ovens is to reheat precooked (such as frozen) food products, rather than to cook uncooked food products in the first instance.
In the past, difficulties have been experienced in various attempts to brown or crispen food products in a microwave oven. A microwave oven heats foods differently from a conventional oven, which cooks the food entirely by radiant heat. Generally speaking, food substances are heated in proportion to their moisture content and absorption of microwave energy, which may result in considerably different heating patterns from those that exist in a conventional oven. Also, this dielectric heating by microwave radiation penetrates into most foods in a way that results in considerably different heating patterns from those that would otherwise be present in a conventional oven. In most cases, microwave energy will heat foods faster than in a conventional oven. For example, a food substance that might require 30 minutes to properly “cook” or reheat in a conventional oven, may take only 3 or 4 minutes to “cook” or reheat in a microwave oven. In a conventional oven, the oven atmosphere is heated to relatively high temperatures to transfer heat to the food surface resulting in the surface always being the hottest area in the food. In a microwave oven, the oven atmosphere is generally not heated; the food itself heats and transfers heat to the surrounding air and in some cases resulting in the outer surface of the food being cooler than the interior or having a soggy surface as the heated interior moisture and steam travel through the outer pastry layers. These differences significantly affect one's ability to brown or crispen a surface of a food product in a microwave oven as the food surface is not exposed to conventional or radiant heat.
To compensate for the microwave oven's inability to crisp and brown the surface of foods, and to prevent foods that are highly absorptive of microwave energy from being overheated, resulting in toughening and dehydration of the food, two specific “microwave active” packaging components have been developed: microwave susceptors and microwave shields.
Microwave shields are devices that do not heat appreciably in response to microwave energy, but reflect virtually all incident microwaves. Metallic foils are generally employed as microwave shields, which has the effect of shielding the food from microwave energy.
In contrast, microwave susceptors are devices that heat in response to microwave energy, converting microwave energy into thermal energy to produce radiant heat that can provide a browning and/or crisping effect to food surfaces placed in contact with the microwave susceptors. Microwave susceptors thus convert a portion of the incident microwave energy into conventional or radiant heat, which assists in cooking the food product's outer surface. Such cooking occurs by any of conduction, convection and/or radiant heating, in addition to the cooking obtained from the microwave radiation itself that has passed through the susceptor laminate. Susceptors may generally comprise almost any portion of the cooking and/or packaging surfaces, and have suitably been employed in the forms of the cooking surfaces of kitchen utensils, the bottom of packaged food products, such as unpopped popcorn, incorporated into folding cartons or trays, and a food wrap for a food product, such as meat-filled sandwiches, bread, and pastry-type products, which when cooked desirably should have a browned or crisped exterior surface.
Because of the above-described problems with browning and crisping foods in microwave ovens, the goal in the art of susceptors had been, and still is, to make the susceptor so that it generates the highest temperatures and most radiant heat possible. This goal was used because generation of the highest temperature possible also tends to mean that the cooking or heating time can be shortened, which is a primary advantage of microwave ovens. Furthermore, when a food load is close to the susceptor surface, heat can be transferred to the food as fast as it is produced by the susceptor.
Susceptors frequently comprise or are included in the packaging for food products as a convenience to the consumer, so that the consumer can simply place the product into a microwave oven without any significant preparation. As a further convenience, such packaging is customarily disposable. Thus, there is a particular need for susceptors that are economical.
However, since susceptors will be brought into contact with foods intended for human consumption, it is necessary to encapsulate the microwave interactive material within films or the like that are approved for contact with food, thus resulting in a multi-layer susceptor product. Such multi-layer products are generally known in the art, and have taken many forms. Customarily, the susceptor product comprises a base sheet such as paper, cellophane, cardboard, box board or the like, a thin layer of microwave interactive material, such as aluminum and/or other selected metals, combinations of metals, alloys and oxides, and a heat resistant barrier film overlying the thin metal layer.
The multi-layer sheet may then be die cut, folded into cartons or pressed into preformed trays or containers and/or decorated with printing to form a package into which food may be inserted by a food processor. Alternatively, the multi-layer sheet may comprise a flexible laminate, which can be formed around a food product as a wrapping material at the food processor's plant. Alternatively, susceptor laminations may be configured as a variety of flexible paper or paper board structures, such as wraps, bags, pouches, sleeves, trays, pads, discs, sleeves, patches, liners, lids, and other designs that would enable the home user and food processor to utilize susceptor materials for their own microwave suitable foods.
The barrier film is typically a polyester (PET) film, due primarily to its heat resistant properties and low cost. However, the barrier film may also be polyimide, cellulose, polyethylene nitrile and other heat resistant films. Its purpose is to provide a functional barrier between the food product and as a carrier for the susceptor metal, and sometimes also to serve as a carrier for a sealable layer to facilitate formation of a package.
The microwave interactive susceptor material is typically a metal, combinations of metals, metal alloy, metal oxide, or derivatives and/or combinations thereof, in single or multi-layer formations, but also may be ceramic or carbon. Any element or compound that absorbs the electromagnetic microwave energy, either electrical and/or magnetic wave forms, and converts it to radiant heat is suitable. The metals are usually applied by using evaporative, sputtering, or electron-beam deposition methods. The metals may also be applied using such suitable methods as printing or gravure processes, and combinations of pre-selected designs of shielding and susceptor patterns and designs can be manufactured by controlled acid etching. Flakes and slurries of susceptor materials, which may be controlled acid etched to provide variations in susceptor radiant heat, are sometimes applied in a rotary printing process. Ceramics and carbon may also be applied in a rotary printing process.
Typically, the susceptor is formed by depositing a film or layer of the microwave interactive material onto the barrier film, e.g., a web of polyester film, followed by laminating the metallized film onto a web of supporting substrate material, usually board, paper or cellulose.
Numerous variations of susceptor materials and manufacturing methods have been proposed and disclosed in the art.
For example, U.S. Pat. No. 4,641,005 to Seiferth discloses a disposable food receptacle for use in microwave cooking, which includes a thin layer of an electrically conductive material, such as an elemental metal such as aluminum, to brown the exterior of the food product. The electrically conductive material is formed as an extremely thin film deposited on a substrate protective layer by a process of a vacuum vapor deposition.
U.S. Pat. No. 5,614,259 to Yang et al. teaches a microwave interactive susceptor in end product condition or form, which are produced by a continuous in-line production method. In the production method, under continuous vacuum, a paper or board substrate is first coated with a thin film of monomer that is cured to a polymer, a metal or other microwave interactive susceptor material is vapor or sputter deposited onto the polymer film, either in an overall layer or preselected pattern, and a thin film monomer is deposited over the susceptor layer and cured or polymerized. The result is described to be an end product ready for use. The process is described to avoid the previously required polyester substrate, and can be made without requiring lamination of a metallized film to a paper or board backing.
U.S. Pat. No. 5,164,562 discloses a combined microwave susceptor/microwave shield packaging product. The packaging comprises at least two spaced susceptor layers in overlying relation. The packaging is described as providing a food packaging/cooking product that keeps the inside of the food product moist without drying out, while heating the surface of the product to a high enough temperature to brown the surface.
U.S. Pat. No. 4,927,991 to Wendt et al. similarly teaches a food package for microwave oven use including a susceptor material in combination with a grid. The combination of the grid and susceptor are described as providing a heating element, which substantially maintains its reflectance, absorbance and transmittance during microwave cooking. The patent describes that this grid and susceptor combination provides substantial uniformity of heating.
However, despite the numerous approaches that have been made to susceptor materials, a common problem with the susceptors is that during use, i.e., during cooking, excess moisture, grease and oil from the food product collect at the surface of the susceptor (polyester film), and thus at the outer surface of the food product. Accordingly, the food product sits in a puddle of food liquids. The result is a soggy food product that has decreased appeal to the end user. The presence of the excess moisture, grease and oil can also deteriorate the food quality, since it results in the food product having a different texture, taste, and appearance from what is expected. The art has recognized this problem that the susceptor materials do not adequately absorb excess grease and oil from food products during cooking.
To overcome this difficulty, the art has proposed the use of nonwoven materials as grease absorbing elements in microwave food packaging. For example, the references “Why the Heat-and-Eat Market is Really Cooking,” Business Week, Jun. 27, 1988, and “Microwave Packet Broils Bacon Cleanly,” Packaging Technology, July 1988, report the introduction of packaging by Wright Brand Foods Inc. and Geo. A. Hormel & Co. developed by 3M Corp. for browning bacon. The package includes an expandable plastic bag and absorbent MICROINSORB® 3M nonwoven pad. Steam generated during cooking of the bacon is contained in the bag to limit evaporative cooling. A high temperature and crisping is effected by the microwave excitation of bacon grease in the package. It is asserted that the nonwoven MICROINSORB® pad also absorbs grease to provide a quality product.
The Business Week reference reports, however, that the product has limited application in browning food products, because steam is not an appropriate medium for browning food products and most foods do not have the high grease content of bacon. Moreover, it is desirable to provide a packaging material that has application for cooking of packaged and non-prepackaged foods.
U.S. Pat. No. 5,124,519 discloses a microwave susceptor composite that includes a first layer of polymeric fibrous material, and a second layer of thermoplastic polymer material having microwave susceptive characteristics. The polymeric susceptor and polymeric fibrous material layers are arranged in lamina surface-to-surface contact and bonded into an integral structure by application of heat and pressure. A uniform composite material is obtained by carding the polymeric fibrous materials prior to bonding the first and second layers. In microwave cooking applications, the fibrous layer of the composite absorbs excess effluents from the food product. Browning and crisping of food products is effected by microwave excitation of the metallized second layer of the composite.
U.S. Pat. No. 5,414,248 discloses an insert useful in a microwavable food container comprising a metallized layer of heat susceptor thickness or a plastic layer, having openings that are in a position to be adjacent to food in the container, a layer of absorbent material comprising fibers, and a substrate layer that is stable to microwave heating conditions. The inserts can also comprise fibers that are capable of spontaneously transporting water or n-decane on the surface thereof.